Field of the Invention
The present invention relates to the field of surface treated optically variable pigment. More particularly, the present invention relates to pacification of optically variable pigments for use in waterborne coating compositions.
The use of optically variable pigments has been described in the art for a variety of applications, such as inks for counterfeit-proof applications such as currency, and generically for coating compositions. They are described, for example, in U.S. Pat. Nos. 4,434,010, 4,704,356, 4,779,898, 4,838,648, 4,930,866, 5,059,245, 5,135,812, 5,171,363, and 5,214,530.
Optically-variable thin-film pigment flakes are prepared by depositing onto a flexible web combinations of transparent dielectric layers, semi-opaque metal layers, and metal reflecting layers to form a multilayer thin film interference structure. The interference structure typically has at least one metal reflecting layer, at least one transparent dielectric layer and at least one semi-transparent metal layer. Various combinations of these layers can be utilized to achieve the desired optically variable effect. In a preferred embodiment, the interference structure produces a dichroic optical effect and has in order, on one side of the metal reflecting layer, at least one transparent dielectric layer and at least one semi-transparent metal layer. In a particularly preferred embodiment, this layer structure is symmetrical on both sides of the metal reflecting layer.
Aluminum is often used as the metal reflecting layer for a variety of reasons such as its cost and commercial availability, although other materials, such as gold, copper, or silver can also be used. The semi-opaque metal layer can be formed from metals such as chromium or nickel. The transparent dielectric layers can be formed from materials such as silicon dioxide, magnesium fluoride, or aluminum oxide. Layer thicknesses can be varied according to the particular desired characteristics of the pigment. For example, U.S. Pat. No. 5,135,812 describes useful thicknesses being on the order of 80 nm for the metal reflecting layer, 5 nm for the semi-opaque metal layers, and thicknesses of a plurality of halfwaves of the particular design wavelength for the transparent dielectric layers.
As mentioned above, the optically-variable thin-film pigment flakes are prepared by coating the semi-transparent metal layers, transparent dielectric layers, and metal reflecting layer onto a flexible web, and separating the web from the multilayer structure to form the pigment flakes. The web is typically a polymer material, such as polyvinyl alcohol or polyethyleneterephthalate. The separation can be accomplished by stripping the multilayer structure from the web, in which case a stripping layer, as is known in the art, is preferably deposited onto the web prior to the other layers. Heat and/or solvent may be used to facilitate the stripping process. Alternatively, the web may be dissolved in a suitable solvent (aqueous or organic, depending on the solubility of the web material) to accomplish the separation. The coated web may optionally be cut or shredded to more easily fit into a container prior to the dissolution step.
As the multilayer structure is separated from the web, it typically breaks into flakes or irregular shapes and sizes. These flakes will usually require further processing to achieve the size requirements for use in a coating composition. This can be accomplished by techniques known in the art, such as ultrasonic agitation, milling, or grinding. It may be preferred to use various combinations of solvents, surfactants, and/or resins during the size reduction process, as is known in the art. The optically variable pigments are capable of producing dramatic visual effects, including dichroic effects not observed in other types of pigments.
The present invention relates to compounds that are useful for surface modification and corrosion inhibition of optically variable pigment particles used in waterborne coating compositions, particularly automotive coating compositions. The invention further relates to decorative coatings formed from aqueous compositions containing surface modified optically variable pigments.
Waterborne automotive paints are gaining widespread usage in the automotive industry due to concerns over organic solvent emissions during application of paints. The new waterborne paints have the disadvantage of using a medium which is corrosive to metallic pigments, such as the optically variable pigments. For example the pH of the waterborne acrylic coating compositions typically ranges from 8.0-9.0, and the polyurethane coating compositions have a pH typically ranging from 7.5 to 8.0. In a basic pH environment, the optically variable pigment is oxidized. The oxidation is a form of corrosion which attacks the exposed metallic surfaces of the optically variable pigment particles. Oxidation of the exposed metallic surfaces affects the light absorbing quality of the chromium and the reflecting quality of the reflective layer. Oxidation of the metallic layers results in the evolution of hydrogen gas. The amount of hydrogen gas evolved is indicative of the amount of oxidation (i.e. corrosion) of the pigment. The hydrogen gas may accumulate in the paint. Furthermore, when a paint with oxidized optically variable pigments is coated onto a substrate, the coating shows discoloration, a significant loss of chroma, a shift in hue and diminished metallic effect.
Deterioration of optically variable pigment may accelerate over time due to continuous contact with the basic pH environment of the coating composition. Coating compositions containing the optically variable pigments are often stored for 6 months or more before application, which can result in significant corrosion of the pigment. If this corrosion remains unchecked the coating composition may be unusable.
Following application of the coating containing optically variable pigment to a substrate, color shift may occur in the coating when exposed to humidity, due to swelling of the dielectric layer of the pigment.
Treatment of optically variable pigment with the compounds of the present invention reduces oxidation of the pigment in waterborne paints. The surface modification of the optically variable pigment is also effective to reduce color change of cured paint films upon exposure to humidity, by protecting the dielectric layers in the pigment. Due to the multi-layer construction and the multi-metal content of the pigment, it was unexpected that surface treated pigments according to the present invention would retain the dichroic effect, and demonstrate decreased gassing and improved humidity resistance.